The present invention relates to a plain bearing for internal combustion engines, in which a back metal layer is lined with an aluminum-base bearing alloy layer, and more particularly to the plain bearing of which coefficient of friction is reduced while maintaining good fatigue resistance property and which is improved in anti-seizure property.
Plain bearings with an aluminum-base bearing alloy are excellent in anti-seizure property, fatigue resistance and wear resistance, and widely used in internal combustion engines for automobiles and general industrial machinery.
Such aluminum-base bearing alloys used in plain bearings for internal combustion engines contain additives such as Sn, Cu and Si for the improvement of their bearing properties. Sn is a soft metal, which contributes to ensuring conformability by virtue of its deformability. Cu strengthens the Al matrix and improves fatigue resistance property. Si serving as hard particles laps a surface of a mating shaft supported by the plain bearing thereby ensuring an lubricant oil film between an inner surface of the plain bearing and the mating shaft to improve anti-seizure property.
While internal combustion engines today have been so designed to meet requirements for ever higher speed, higher power output, lighter weight and lower fuel consumption, the plain bearings are desired to have further improved performance. Thus, such high speed and high power internal combustion engines bring a thinner lubricant oil film in the plain bearing and pose a problem of a susceptible shape change of a lightened bearing housing. As a result, the oil film becomes extremely thinner to increase metal-to-metal contact portions resulting in a possibility of abnormal wear or seizure due to adhesion. In order to avoid such disadvantages, plain bearings are required to have properties of conformability, high seizure resistance and good fatigue resistance, wherein the good conformability is for forming an oil film at a running-in stage of operation, the high seizure resistance is of a countermeasure against a metal-to-metal contact, and the good fatigue-resistance is for preventing occurrence of fatigue at an early stage of operation.
On the other hand, as one way for lower fuel consumption, a lubricant oil having a low viscosity is used in plain bearings in order to reduce shearing resistance of the lubricant oil resulting in that the oil film becomes thinner thereby increasing metal-to-metal contact portions. In such metal-to-metal contact portions, the frictional resistance between a mating shaft and a plain bearing is high, so that the plain bearings might not contribute to lower fuel consumption and friction heat is generated. Because of the friction heat, the viscosity of the lubricant oil may drop to promote the metal-to-metal contact. In order to prevent such heat generation, it is required to make the coefficient of friction of the plain bearing lower with relation to the mating shaft. If the coefficient of friction can be made lower, it is possible not only to reduce heat generation but also to improve anti-seizure and fatigue resistance properties.
PCT WO 02/040743 A1, which does not relate to a plain bearing, discloses to provide a piston in an internal combustion engine with a surface layer containing MOS2 by shooting a fine powder of molybdenum disulfide (MoS2), as a solid lubricant, to collide against the piston so as to penetrate into the surface layer of the piston within a depth of not more than 20 μm.
It has been well known to reduce the friction resistance of a plain bearing with relation to a mating shaft by adding a solid lubricant in a bearing alloy, or by coating the surface of the bearing alloy with a solid lubricant with utilization of a binder resin.
However, in the case of coating, there are problems of inferior bonding strength thereof and a possibility of an insufficient friction reducing effect of the solid lubricant due to the presence of the binder resin. In the case of the bearing alloy containing the solid lubricant, when an aluminum-base bearing alloy, for example, is produced by the powder metallurgical method, a blend of an aluminum-base bearing alloy powder and a solid lubricant powder is sintered to obtain a bearing which is not durable in actual use in internal combustion engines because of an inferior powder bonding problem resulting in lack of alloy strength.
Accordingly, there might be an idea of applying the technique shown in PCT WO 02/040743 A1 to a plain bearing used in internal combustion engines to provide it with a surface layer containing MOS2. According to the publication, a fine powder of molybdenum disulfide (MOS2) is shot to a piston to form a surface layer containing MOS2 and having a thickness of not more than 20 μm, and to simultaneously provide the surface of the piston with fine dimples, whereby realizing reduction of friction by virtue of the lubrication effect of MoS2 itself and an oil reservoir effect of the dimples on the surface.
The publication teaches further that, when the fine powder of molybdenum disulfide (MOS2) is shot at a high speed to collide against the piston, the surface temperature of the piston rises up to such a high level that the surface layer of the piston is melted partially to form an intermetallic compound derived from alloy components and Mo in MOS2, whereby the anchoring strength of MoS2 is increased, and the surface is work-hardened to have improved wear resistance property.
However, it should be noted that the piston is made of a hard metal having a Vickers hardness of not less than 300 whereas the aluminum-base bearing alloy is soft not like as the piston. From this difference between the aluminum-base bearing alloy and the piston, according to an experiment conducted by the present inventors, when a fine powder of a solid lubricant is shot to collide against the aluminum-base bearing alloy, a surface roughness of the alloy increases, and large dimples each having an irregular shape are formed in the surface of the alloy resulting in occurrence of breaking of an lubricant oil film due to such dimples while overriding the oil reservoir effect by the dimples, whereby occurring heat generation due to an excessive metal-to-metal contact resulting in deteriorated anti-seizure and fatigue resistance properties.